I had a problem with the graphics card in my machine showing jagged objects in 3d viewer and the solution was to turn off just the Enhanced 3D Performance in the manual tune.
This might be a better option if it works for you as it will use the graphics card rather than using the cpu.
If a post provides a fix for your issue, click on "Accept as Solution" to help other users find solutions to problems they might have that are similar to yours.
Andrew Puller
Maitland, NSW, Australia
Windows 10 Enterprise 64bit
Intel core i7 11800 @ 2.30 GHz with 32GB Ram
Civil 3d 2021